A typical chemical circulation of a pulp mill is shown in FIG. 1. Process chemicals are recovered to a char bed of a recovery boiler by burning concentrated black liquor therein. From the char bed smelt should flow in a continuous manner to a dissolving tank. In the dissolving tank, the smelt is dissolved in weak white liquor to produce green liquor.
In a solution known from EP0524743, the sodium carbonate concentration of green liquor is regulated by measuring the conductivity of the green liquor. From measurements, the sodium carbonate concentration is determined, and the flow of weak white liquor can be adjusted to maintain a proper sodium carbonate concentration. From FI114813 it is known that the density of green liquor correlates with total titratable alkali (TTA) of the green liquor.
The concentration of chemicals within the green liquor should be high in order to efficiently recover chemical in subsequent steps of the process. In the document EP0524743, the amount of white liquor fed to the dissolving tank is controlled by using the measured conductivity of green liquor to control the concentration of chemicals in the green liquor.
The aforementioned solution has thus several drawbacks. First, since only the electrical conductivity is measured, the sodium carbonate concentration is not accurate. Second, also other compounds that sodium carbonate affect the process of dissolving. Some compounds may solidify, provided that that the concentration are too high. Third, in the process, in particular sodium sulphide should be recovered, while the content of sodium sulphate is less important. Thus, the reduction of the green liquor should also be maximized. However, by controlling only the flow of weak white liquor, reduction cannot be affected.
The aim of the present invention is to overcome at least some of the drawbacks of prior art.